Pre-AA, base softcaps are very well defined because they used to instead be
hardcaps. Until changed several months into Luclin, AC returns over caps were -0-. This led to rediculous situations like the premier warrior pants being 12ac (but 65! hp). These hard (now soft) caps are raised by the melee mitigation line of AA that began then and continue through the DoD expansion.
Yes, Scowls, as described at
http://samanna.net/gen.info/effects.shtml for a year or more now, there is a point of diminishing returns to be observed against any particular monster when you meet and then pass its ATK rating. This value is so dynamic, though, as to not be worth discovering. It can vary by 100s & 1000s within a single zone, let alone across zones or between level-expanding expansions. With AC, more is better, even past these points of diminishing returns.
Shaman AC hardcap, equal to the current pre-AA base
softcap, was 1100 or 1150. Forgive me, it's been a long time. The only one I remember with absolute surety is the Monk one, at 120 worn, since it is exactly 2x AoW pants. It's concievable that the current softcap is up around 1600, but the exact value is mostly meaningless, since all shaman who devote attention to their gear are well over it.
The exciting thing about AC on shields is that all that softcap nonsense doesn't apply. Every 6ac provides the same delta of protection that you gained putting Banded Bracer over your naked bod. (that's hot stuff) In the hard-to-directly-see world of melee mitigation, shield (not just any item, actual shields) AC in the secondary slot (back slot-shields need not apply) is godlike.
With AC, more is better, and the only way for shaman to access non-kneecapped (oops, I mean non-softcapped!) AC is via secondary slot shields. This is why I'm committed to always wearing an AC augment there.